Effectiveness and mechanism of cadmium (Cd) sorption on original, acidified and ball milling nano-particle red muds were investigated using batch sorption experiments, sequential extraction analysis and X-ray absorption near edge structure (XANES) spectroscopy. The maximum sorption capacity of Cd was 0.16, 0.19, and 0.21mol/kg for the original, acidified, and nano-particle red muds at pH 6.5, respectively. Both acidification and ball-milling treatments significantly enhanced Cd sorption and facilitated transformation of Cd into less extractable fractions. The Cd LIII-edge XANES analysis indicated the formation of inner-sphere complexes of Cd similar to XCdOH (X represents surface groups on red mud) on the red mud surfaces although outer-sphere complexes of Cd were the primary species. This work shed light on the potential application of red mud to remediate Cd-contaminated soils and illustrated the promising tool of XANES spectroscopy for speciation of multicomponent systems of environmental relevance.

China’s strategies to control acidifying pollutants and particulate matter (PM) may be in conflict for soil acidification abatement. Acidifying pollutant emissions are estimated for 2005 and 2020 with anticipated control policies. PM emissions including base cations (BCs) are evaluated with two scenarios, a base case applying existing policy to 2020, and a control case including anticipated tightened measures. Depositions of sulfur (S), nitrogen (N) and BCs are simulated and their acidification risks are evaluated with critical load (CL). In 2005, the area exceeding CL covered 15.6% of mainland China, with total exceedance of 2.2 Mt S. These values decrease in the base scenario 2020, implying partial recovery from acidification. Under more realistic PM control, the respective estimates are 17.9% and 2.4 Mt S, indicating increased acidification risks due to abatement of acid-neutralizing BCs. China’s anthropogenic PM abatement will have potentially stronger chemical implications for acidification than developed countries.

The role of nitrogen (N) in acidification of soil and water has become relatively more important as the deposition of sulphur has decreased. Starting in 1991, we have conducted a whole-catchment experiment with N addition at Gårdsjön, Sweden, to investigate the risk of N saturation. We have added 41kgNha⁻¹yr⁻¹ as NH₄NO₃ to the ambient 9kgNha⁻¹yr⁻¹ in fortnightly doses by means of sprinkling system. The fraction of input N lost to runoff has increased from 0% to 10%. Increased concentrations of NO₃ in runoff partially offset the decreasing concentrations of SO₄ and slowed ecosystem recovery from acid deposition. From 1990–2002, about 5% of the total N input went to runoff, 44% to biomass, and the remaining 51% to soil. The soil N pool increased by 5%. N deposition enhanced carbon (C) sequestration at a mean C/N ratio of 42–59gg⁻¹.

Critical loads are widely used in the effects-based assessment of emission reduction policies. While the impacts of acidification have diminished, there is increasing concern regarding the effects of nitrogen deposition on terrestrial ecosystems. In this context much attention has been focussed on empirical critical loads as well as simulations with linked geochemistry–vegetation models. Surprisingly little attention has been paid to adapt the widely used simple mass balance approach. This approach has the well-established benefit of easy regional applicability, while incorporating specified critical chemical criteria to protect specified receptors. As plant occurrence/biodiversity is related to both the nutrient and acidity status of an ecosystem, a single abiotic factor (chemical criterion) is not sufficient. Rather than an upper limit for deposition (i.e., critical load), linked nutrient nitrogen and acidity chemical criteria for plant occurrence result in an ‘optimal’ nitrogen and sulphur deposition envelope.

Changes in sulphur and nitrogen pollution in Swedish forests have been assessed in relation to European emission reductions, based on measurements in the Swedish Throughfall Monitoring Network. Measurements were analysed over 20 years with a focus on the 12-year period 1996 to 2008. Air concentrations of SO₂ and NO₂, have decreased. The SO₄-deposition has decreased in parallel with the European emission reductions. Soil water SO₄-concentrations have decreased at most sites but the pH, ANC and inorganic Al-concentrations indicated acidification recovery only at some of the sites. No changes in the bulk deposition of inorganic nitrogen could be demonstrated. Elevated NO₃-concentrations in the soil water occurred at irregular occasions at some southern sites. Despite considerable air pollution emission reductions in Europe, acidification recovery in Swedish forests soils is slow. Nitrogen deposition to Swedish forests continues at elevated levels that may lead to leaching of nitrate to surface waters.

Soil acidification was assessed in the Eastern Sudetes Mountains (Czech Republic) between 1941 and 2003, i.e. before and after the period of major industrial pollution (1950s–1990s). The twenty sites included in our study were distributed along a gradient of altitude ranging 1000m. Values of pH have decreased in 80–90% of the pairs of samples after the six decades, on average by 0.7 for pH-H₂O and 0.6 for pH-KCl. Organic matter increased in the topsoil, probably reflecting a change in decomposition conditions. The most important finding is that the acidification varies along the joint gradient of altitude/tree layer composition, and displays a changing pattern in three soil horizons (A, B and C). Contrary to expectations, most acidified were soils in beech forests at lower elevations.

The effect of pH ranging from 8.0 to 6.8 (total scale – pHT) on fertilization, cleavage and larval development until pluteus stage was assessed in an intertidal temperate sea urchin. Gametes were obtained from adults collected in two contrasting tide pools, one showing a significant nocturnal pH decrease (lowest pHT=7.4) and another where pH was more stable (lowest pHT=7.8). The highest pHT at which significant effects on fertilization and cleavage were recorded was 7.6. On the contrary, larval development was only affected below pHT 7.4, a value equal or lower than that reported for several subtidal species. This suggests that sea urchins inhabiting stressful intertidal environments produce offspring that may better resist future ocean acidification. Moreover, at pHT 7.4, the fertilization rate of gametes whose progenitors came from the tide pool with higher pH decrease was significantly higher, indicating a possible acclimatization or adaptation of gametes to pH stress.

This paper compares lake chemistry in the Adirondack region of New York measured by the Temporally Integrated Monitoring of Ecosystems (TIME) and Adirondack Long-Term Monitoring (ALTM) programs by examining the data from six lakes common to both programs. Both programs were initiated in the early 1990s to track the efficacy of emission reduction policies and to assess the full impacts of acid deposition on surface water chemistry. They now serve to inform on the emerging chemical recovery of these waters. The Adirondack TIME program utilizes a probability-based approach to assess chronic acidification in a population of lakes using one summer sample per year. The ALTM attempts to track changes in both chronic and episodic acidification across a gradient of lake types using monthly samples. The ALTM project has two important attributes that contrast with the TIME program in the Adirondacks: higher temporal resolution (monthly versus once during the summer or fall) and speciation of aluminum. In particular, the ALTM program provides inorganic monomeric aluminum (AlIM), the fraction of Al that is most toxic. The monthly sampling of the ALTM program includes the spring snowmelt period when acid-neutralizing capacity and pH are near their lowest and Al levels are near their highest. We compare chemistry trends (1992–2008) for sulfate, nitrate, base cations, dissolved organic carbon, hydrogen ion, acid neutralizing capacity, and Al for the six lakes common to both programs. We also compare relatively high springtime AlIM concentrations from the ALTM with relatively low summertime total Al concentrations from the TIME, showing that the ALTM program provides a more biologically relevant indicator of the effects of acid deposition, illustrating the value of the complementary monitoring efforts in the Adirondack region.

Nitrogen dioxide is an important gaseous air pollutant. It plays a major role in atmospheric chemistry, particularly in the formation of secondary air pollutants, and contributes to environmental acidification. A comprehensive assessment of NO2 levels in the atmosphere is required for developing effective strategies for control of air pollution and air quality improvement. Air pollution is a serious problem in all major cities of Pakistan and needs to be addressed to minimize detrimental effects on human health and urban vegetation. In this research, we focused on the monitoring of NO2 levels in the urban environment of Rawalpindi city. Because of the lack of expensive continuous sampling devices and to get a good spatial coverage of the NO2 concentrations in the study area, NO2 passive samplers were exposed at 42 different sites within the city limits of Rawalpindi from January to December, 2008. Samplers were exchanged every 10 days and the associated meteorological conditions like temperature, wind speed, rainfall and relative humidity were also monitored. The average NO2 concentration was found to be 27.46 ± 0.32 ppb. The highest values of NO2 were measured near to main roads and educational institutions due to intense flow of road vehicles. Moreover, the study showed that the values obtained for NO2 for all sampling points exceeded the annual limit value set by World Health Organization. So, this is very important to take different steps to control this before it becomes a serious hazard for people living in those areas.

The recovery potential of stream acidification from years of acidic deposition is dependent on biogeochemical processes and varies among different acid-sensitive regions. Studies that investigate long-term trends and seasonal variability of stream chemistry in the context of atmospheric deposition and watershed setting provide crucial assessments on governing biogeochemical processes. In this study, water chemistries were investigated in Noland Divide watershed (NDW), a high-elevation watershed in the Great Smoky Mountains National Park (GRSM) of the southern Appalachian region. Monitoring data from 1991 to 2007 for deposition and stream water chemistries were statistically analyzed for long-term trends and seasonal patterns by using Seasonal Kendall Tau tests. Precipitation declined over this study period, where throughfall (TF) declined significantly by 5.76 cm year−1. Precipitation patterns play a key role in the fate and transport of acid pollutants. On a monthly volume-weighted basis, pH of TF and wet deposition, and stream water did not significantly change over time remaining around 4.3, 4.7, and 5.8, respectively. Per NDW area, TF SO4 2- flux declined 356.16 eq year−1 and SO4 2- concentrations did not change significantly over time. Stream SO4 2- remained about 30 μeq L−1 exhibiting no long-term trends or seasonal patterns. SO4 2- retention was generally greater during drier months. TF monthly volume-weighted NH4 + and NO3 - concentrations significantly increased by 0.80 μeq L−1 year−1 and 1.24 μeq L−1 year−1, respectively. TF NH4 + fluxes increased by 95.76 eq year−1. Most of NH4 + was retained in the watershed, and NO3 - retention was much lower than NH4 +. Stream monthly volume-weighted NO3 - concentrations and fluxes significantly declined by 0.56 μeq L−1 year−1 and 139.56 eq year−1, respectively. Overall, in NDW, inorganic nitrogen was exported before 1999 and retained since then, presumably from forest regrowth after Frazer fir die-off in the 1970s from balsam wooly adelgid infestation. Stream export of NO3 - was greater during winter than summer months. During the period from 1999 to 2007, stream base cations did not exhibit significant changes, apparently regulated by soil supply. Statistical models predicting stream pH, ANC, SO4 2-, and NO3 - concentrations were largely correlated with stream discharge and number of dry days between precipitation events and SO4 2- deposition. Dependent on precipitation, governing biogeochemical processes in NDW appear to be SO4 2- adsorption, nitrification, and NO3 - forest uptake. This study provided essential information to aid the GRSM management for developing predictive models of the future water quality and potential impacts from climate change.